The present invention relates generally to controlling temperature of a fluid in a refrigeration system. More specifically, the present invention relates to using fuzzy logic to control the rate at which the leaving chilled water temperature of a chiller is pulled down to a predetermined set point while minimizing overshoot of the predetermined setpoint.
In a chilled water system, chilled water is circulated through a building to remove heat from the building. The water in the chilled water system is cooled in an evaporator or chiller using a heat exchanger, wherein the water is cooled by a refrigerant that accepts heat from the water. Chilled water systems are often operated, for optimum efficiency, at temperatures (operating setpoints) within a few degrees of the freezing point of water. In existing chillers, a control system is often programmed to shut down the chiller as soon as the evaporator water temperature decreases to a certain temperature (a cutout point) near or below the freezing point of water to avoid freezing the water tubes and damaging the chiller. For example, a chiller may have an operating setpoint of 35xc2x0 F. and a cutout point of 34xc2x0 F.
A difference of only one degree between the operating setpoint and the cutout point in the chiller, generally does not cause problems during normal or steady-state operation of the chiller. However, when the chiller is required during a pull-down to reduce the water temperature from an ambient temperature to the operating setpoint, a one degree difference between the operating setpoint and the cutout point can be problematic. The chiller may be shut-down by the control system during a pull-down, if the water temperature in the chiller overshoots the operating setpoint and reaches the cutout point. To avoid this problem, it is necessary that chiller control systems be programmed to minimize the pull-down overshoot without sacrificing the pull-down response time.
In standard proportional-integral-derivative (PID) controls, the derivative term controls the rate of approach to the temperature setpoint and is manually tuned and then fixed to achieve a desired maximum overshoot/undershoot for worst case conditions. However, as system dynamics change due to load variations, etc., the response time may become too slow under certain conditions due to the fixed derivative (rate) term. One way to avoid the problems associated with the fixed derivative term is to use a self-tuning control which automatically adjusts the rate term to maintain the desired response time while keeping overshoot/undershoot within acceptable limits. These self-tuning algorithms require knowledge of the amount of undershoot/overshoot before making appropriate adjustments to compensate the control. However, these algorithms may result in the chiller shutting down during the initial and subsequent pull-downs, since the control has not yet completely compensated for the initial overshoots.
Therefore, what is needed is a fuzzy logic control algorithm that can maintain a desired pull-down response time and minimize or eliminate overshoot of an operating setpoint by adjusting a rate sensitivity coefficient of the control algorithm during a mode of pull-down operation that is commenced when the water temperature reaches a few degrees above the operating setpoint.
One embodiment of the present invention is directed to a temperature control system for a refrigeration system. The refrigeration system comprises a compressor, a condenser, and an evaporator all connected in a closed refrigeration circuit. The compressor has a plurality of inlet guides vanes that are adjustable and controlled by an actuator. The temperature control system includes a sensor for detecting the temperature of the leaving fluid in the evaporator and for providing a temperature signal. The temperature control system also includes a microprocessor that samples the temperature signal at a predetermined interval during a pull-down operation on the leaving fluid temperature in the evaporator. The microprocessor then generates a control signal for the actuator by applying the temperature signal to a fuzzy logic control algorithm configured to minimize or eliminate overshoot of a setpoint temperature of the leaving fluid temperature of the evaporator during the pull-down operation.
Another embodiment of the present invention is directed to a method of calculating a control signal to control flow of refrigerant in a refrigeration system during an operation to adjust the temperature of a secondary refrigerant in the refrigeration system to a predetermined setpoint temperature. The method includes the step of generating a temperature signal for the secondary refrigerant in the refrigeration system. An error signal is generated using the temperature signal and a rate signal is generated using the error signal. A control signal is generated using fuzzy logic reasoning in response to the error signal, the rate signal and a rate sensitivity signal to control the flow of refrigerant in the refrigeration system. Finally, the flow of refrigerant in the refrigeration system is controlled to adjust the temperature of the secondary refrigerant in the refrigeration system to minimize or eliminate overshoot of the predetermined setpoint temperature in response to the control signal.
Yet another embodiment of the present invention is directed to a method of calculating a control signal for an actuator of a plurality of vanes to control flow of refrigerant from an evaporator during a pull-down operation to adjust a temperature of leaving water from the evaporator to a predetermined setpoint temperature. The method comprises the steps of measuring, at predetermined intervals, a temperature of the leaving water from the evaporator. An error value is calculated from the difference between the measured temperature and the predetermined setpoint temperature and a pull-down rate value is calculated from a difference between the error value and a previously calculated error value from the prior interval. Next, a control signal is generated at each predetermined interval using a fuzzy logic control algorithm in response to the error value, the pull-down rate value and an adjustable rate sensitivity value. Finally, the actuator is operated in response to the control signal to position the plurality of vanes to control flow of refrigerant from the evaporator to minimize or eliminate overshoot of the predetermined setpoint temperature during adjustment of the temperature of the leaving water from the evaporator to the predetermined setpoint temperature.
Still another embodiment of the present invention is directed to a refrigeration system comprising a compressor, a condenser, and an evaporator connected in a closed refrigeration circuit. A refrigerant is circulated through the refrigeration circuit. Furthermore, the compressor has a plurality of inlet guides vanes adjustable by an actuator. The refrigeration system also includes a sensor for detecting a leaving fluid temperature from the evaporator to provide a temperature signal and a microprocessor generating an error signal, an error rate of change signal and an adjustable rate sensitivity signal at a predetermined interval during a pull-down operation on the leaving fluid temperature in the evaporator in response to receiving the temperature signal. The microprocessor also generates a control signal for the actuator of the plurality of inlet guide vanes using a fuzzy logic control algorithm. The fuzzy logic control algorithm has the error signal, the error rate of change signal and the adjustable rate sensitivity signal as inputs. The control signal is used to control refrigerant flow from the evaporator to minimize or eliminate overshoot of a setpoint temperature for the leaving fluid temperature of the evaporator during the pull-down operation.
A further embodiment of the present invention is directed to computer program product embodied on a computer readable medium and executable by a microprocessor for calculating a control signal for an actuator of a plurality of vanes to control flow of refrigerant from an evaporator during a pull-down operation to adjust a temperature of leaving water from the evaporator to a predetermined setpoint temperature. The computer program product comprising computer instructions for executing the steps of measuring, at predetermined intervals, a temperature of the leaving water from the evaporator. The computer program product also comprises computer instructions for executing the steps of calculating an error value from the difference between the measured temperature and the predetermined setpoint temperature, calculating a rate value from a difference between the error value and a previously calculated error value from a prior interval, and calculating an adjustable rate sensitivity value using a fuzzy logic algorithm. In addition, the computer program product comprises computer instructions for executing the step of generating a control signal at each predetermined interval using a fuzzy logic control algorithm in response to the error value, the rate value and the adjustable rate sensitivity value. The control signal, when transmitted to the actuator, operates the actuator to position the plurality of vanes to control flow of refrigerant from the evaporator to minimize or eliminate overshoot of the predetermined setpoint temperature during adjustment of the temperature of the leaving water from the evaporator to the predetermined setpoint temperature.
One advantage of the present invention is that a control algorithm is used to minimize or eliminate pull-down overshoot without sacrificing response time.